Comparison of simulated forest responses to biosolids applications

Organic matter and N are added to humus pools of the LINKAGES simulator of forest growth and N cycling at a range of application rates to investigate long-term effects of biosolids (sewage sludge) on forest productivity. Two conifer plantations (Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco vat. menziesii], loblolly pine [Pinus taeda L.]) and a northern hardwood forest located in contrasting climatic regions are investigated. Single applications of biosolids are given at 0, 5, 10, 20, and 40 Mg/ha, and multiple applications are given on seven occasions at 3-yr intervals at rates of 5 and 10 Mg/ha. Highly significant increases in aboveground phytomass and net primary productivity of Douglas-fir plantations are obtained in a 100-yr simulation with increasing biosolids application rates. Results for loblolly pine from a 50-yr simulation produced about half the growth response of Douglas-fir. Long-term simulations of northern hardwoods showed modest growth responses and small increases in NPP with added biosolids. The phytomass of one overstory and three understory species in the hardwood forest changed in response to different biosolids applications and varying species sensitivity to N supply. Biosolids are a significant resource for enhancing forest productivity, particularly in conifer plantations. Estimates of N leaching losses from simulated forest sites combined with a literature review of leaching losses suggest that biosolids applications at 3-yr intervals with rates less than 8.5 Mg/ha (0.4 Mg N/ha) during active forest growth may pose little off-site contamination risk to ground water or surface waters.